More attention has been paid to environmental issues in recent years, and a low-pollution water base paint without containing an organic solvent has attracted attention. In addition, with noticeable technical development in water base paints, appearance of high-grade finishing that could conventionally be achieved only by a solvent-type paint can now be realized also by a water base paint. In many metallic pigments (also referred to as metal pigments), however, a film obtained by blending the pigments in a water base paint has particularly been low in chemical resistance and water (moisture) resistance, and few practically usable water base paints have been available.
Chemical resistance of a film refers to such a property that, even when a film is immersed in an aqueous solution of a chemical agent such as acid, alkali or salt, various characteristics such as a color tone and the like are less likely to vary. This chemical resistance of a film containing metallic pigments is mainly dependent on capability of the metallic pigments. Therefore, in order to improve chemical resistance of a film formed by a paint containing metallic pigments, Japanese Patent Laying-Open No. 62-081460 (Patent Document 1) proposes a metal pigment coated with a copolymer of trimethylolpropane triacrylate and/or trimethylolpropane trimethacrylate and a small amount of acrylic acid and/or methacrylic acid. Japanese Patent Laying-Open No. 62-253668 (Patent Document 2) proposes a resin-coated metal pigment characterized in that a surface thereof is coated in a firmly adhered manner with a resin having highly three-dimensional structure and generated from radical polymeric unsaturated carboxylic acid and/or phosphomonoester or phosphodiester having radical polymeric double bond, and a monomer having three or more radical polymeric double bonds, that ΔE showing evaluation of alkali resistance is not higher than 1.0, and that the pigment does not substantially aggregate in a heat resistance and thermal stability test. Japanese Patent Laying-Open No. 64-040566 (Patent Document 3) proposes an aluminum flake evenly coated with a copolymer obtained as a result of reaction of at least two selected from the group consisting of an oligomer and a monomer having at least one polymeric double bond and having a microscopically smooth surface. Japanese Patent Laying-Open No. 2005-146111 (Patent Document 4) proposes a resin-coated aluminum pigment in which a surface of a raw material aluminum pigment is coated with a copolymer obtained by polymerizing a monomer having polymeric double bond, a monomer having one polymeric double bond and one benzene ring, and (meth)acrylic acid. Japanese Patent Laying-Open No. 2007-119671 (Patent Document 5) proposes a metal pigment in which phosphoric ester having radical polymeric double bond is adsorbed as a result of contact with the metal pigment and thereafter a resin-coated layer is formed by polymerization of a monomer having polymeric double bond.
In addition, water (moisture) resistance of a film refers to such a property as maintaining corrosion resistance capability that alteration of a color tone or the like of the film is prevented even when the film is held in water at a temperature relatively higher than room temperature (approximately from 40 to 50° C.) or in a moist state at a relative humidity not lower than 98%. This water (moisture) resistance of a film containing metallic pigments is also mainly dependent on capability of the metallic pigments. Therefore, in order to improve water (moisture) resistance of a film formed by a water base paint containing metallic pigments, Japanese Patent Laying-Open No. 2004-131542 (Patent Document 6) proposes an aluminum pigment coated with a hydrolytic condensate of a compound having siloxane bond. Japanese Patent Laying-Open No. 2007-204692 (Patent Document 7) proposes an aluminum pigment coated with hydrolytic condensate of a compound having siloxane bond and a resin composed of at least one selected from radical polymeric unsaturated carboxylic acid and/or phosphomonoester or phosphodiester or phosphoric acid monoester or phosphoric acid diester having radical polymeric double bond and a monomer having two or more radical polymeric double bonds.
According to these techniques, though market's demands are met to some extent in applications of an organic-solvent-based paint, capability of a film is insufficient in applications of a water base paint. This may be attributed to fundamental difference in resin and solvent between the paints, and in any case, higher capability of a film is required in a water base paint.
WO2006/064652 (Patent Document 8) proposes a metallic pigment including an aluminum particle as a base particle and having a coating layer constituted of a single layer or a plurality of layers for coating a surface of the aluminum particle formed thereon, in which an outermost layer of the coating layer contains a polymer obtained as a result of polymerization reaction of a monomer having a basic group and at least one polymeric double bond, and also proposes a metallic pigment having a surface modifier layer formed outside the coating layer.
According to this Patent Document 8, since the coating layer is formed in order to provide the metallic pigment further with corrosion resistance (paragraph 0042 of Patent Document 8), it is understood that the surface modifier layer serving as an outermost surface layer of the coating layer is formed literally in a layered manner.
In such Patent Document 8, the reason why the outermost layer located directly under the surface modifier layer contains a polymer obtained as a result of polymerization reaction of a monomer having a basic group and at least one polymeric double bond is to form a basic point serving as an adsorption site for a phosphate group of the surface modifier layer on the surface of the outermost layer, and thus firm adsorption between the surface modifier layer and the outermost layer attributed to acid-base interaction can be achieved (the same paragraph). Such a feature can exhibit an advantage that corrosion resistance can satisfactorily be maintained if the metallic pigment is included in a powder paint. On the other hand, if this metallic pigment is blended in a water base paint, water (moisture) resistance thereof was not sufficient for practical use, although its chemical resistance was similarly good. Therefore, further improvement in water (moisture) resistance with chemical resistance being maintained has been demanded.
In general, chemical resistance does not tend to be improved by surface treatment for providing water (moisture) resistance, whereas water (moisture) resistance does not tend to be improved by surface treatment for providing chemical resistance. Therefore, it has been difficult to obtain a pigment achieving both of chemical resistance and water (moisture) resistance at a level sufficient for practical use.